How to execute async functions in order using pure js

stepAsyncFuncs.js

// Executes async functions in order using pure js

/**
 * Loop async func in order
 * @param {Array} - array each object has below properties:
 *  - fn: {Function} - function to loop
 *  - param: {Object} - parameter to pass the function
 * @param {Function} - error-first style callback returns below params as argument:
 *  - errs: array of error
 *  - results: array of result
 */
function stepAsyncFuncs(arr, cb) {
  let i = 0;
  let len = arr.length;
  let counter = 0;
  let errs = [];
  let results = [];
  if (!arr || arr.length <= 0)
    return cb && cb(errs, results);

  function _stepAsyncFuncs(arr, cb) {
    let fn = arr[i]["fn"];
    let param = arr[i]["param"];
    setTimeout(function() {
      fn(param, function(err, msg) {
        // console.log("err:", err);
        errs.push(err);
        results.push(msg);
        i += 1;
        counter += 1;
        if (counter >= len) {
          return cb && cb(errs, results);
        } else {
          _stepAsyncFuncs(arr, cb);
        }
      });
    }, 1);

  }
  // fire at first:
  _stepAsyncFuncs(arr, cb);
}



// Any async function:
let asyncFn = function({
  no,
  time
}, cb) {
  setTimeout(function() {
    let msg = "I am no." + no;
    console.log(msg);
    return cb(null, msg);
  }, time);
}

// Make step list:
let steps = [{
  fn: asyncFn,
  param: {
    no: 0,
    time: 2000
  }
}, {
  fn: asyncFn,
  param: {
    no: 1,
    time: 100
  }
}, {
  fn: asyncFn,
  param: {
    no: 2,
    time: 1000
  }
}, {
  fn: asyncFn,
  param: {
    no: 3,
    time: 300
  }
}];

// Executes  all async functions in order following the step list:
stepAsyncFuncs(steps, function(errs, results) {
  console.log("All execution has end!");
  console.log(" errs:", errs);
  console.log(" results:", results);
});

/* Console log:
I am no.0
I am no.1
I am no.2
I am no.3
All execution has end!
  errs: [null, null, null, null]
  results: ["I am no.0", "I am no.1", "I am no.2", "I am no.3"]
*/

stepAsyncFuncsUsingGenerator.js

// Executes async functions in order using pure js (uses Generator)

// Any async function:
let asyncFn = function(iter, {
  no,
  time
}, cb) {
  // Unless use `setTimeout`, `Generator is already running` error occurs!
  setTimeout(function() {
    let msg = "I am no." + no;
    console.log(msg);
    cb(null, msg);
    return iter.next();
  }, time);
}

// // sleep
// function sleep(iter, ms) {
//   setTimeout(function() {
//     iter.next();
//   }, ms);
// }

// Make step list:
let steps = [{
  fn: asyncFn,
  param: {
    no: 0,
    time: 100
  }
}, {
  fn: asyncFn,
  param: {
    no: 1,
    time: 40
  }
}, {
  fn: asyncFn,
  param: {
    no: 2,
    time: 1000
  }
}, {
  fn: asyncFn,
  param: {
    no: 3,
    time: 10
  }
}];

function stepAsyncFn(steps, cb) {
  let errs = [],
    results = [];
  let iter = function*() {
    let fn, param;
    for (let len = steps.length, i = 0; i < len; i++) {
      fn = steps[i]["fn"];
      param = steps[i]["param"];
      yield fn(iter, param, function(err, result) {
        // console.log("fn callback err:", err, "result:", result);
        errs.push(err);
        results.push(result);
      });
      // console.log("OK, sleeping now...");
      // yield sleep(iter, 2000);
    }
    return cb && cb(errs, results);
  }();
  iter.next(); // Fire!
}

// Execute!
stepAsyncFn(steps, function(errs, results) {
  console.log("all fn complete errs:", errs, "results:", results);
});

/* Console log:
I am no.0
I am no.1
I am no.2
I am no.3
all fn complete 
 errs: [null, null, null, null]
 results: ["I am no.0", "I am no.1", "I am no.2", "I am no.3"]
*/